Ocular lenses are worn by many people to correct vision problems. Vision problems are caused by aberrations of the light rays entering the eyes. These include low order aberrations, such as myopia, hyperopia, and astigmatism; and higher order aberrations, such as spherical, coma, trefoil; and chromatic aberrations. Because the distortion introduced by aberrations into an optical system significantly degrades the quality of the images on the image plane of such system, there are advantages to the reduction of those aberrations.
Ocular lenses are typically made by generating prescriptions in lens blanks. This is accomplished by altering the topography of the surface of a lens blank.
Recently, attention has been given to methods of generating low order prescription in lens blanks using a patient's measured wavefront information. Currently, several techniques can be utilized to determine the optimum low order refraction from the high order, including: the Gaussian Least Squares Fit, point spread optimization, and neural network analysis. Some of these techniques may be employed to not only derive the best low order prescription from the high order values, but may also be used to “fit” an optimum wavefront across an entire spectacle lens based on the patient's measured wavefront.
Using one or more of these fitting techniques may yield a better refraction than conventional subjective refractions in the center zone, but consideration must be given to off-axis gaze angles. In particular, one considerable disadvantage of traditional lens manufacturing is that that many people experience distortion when looking off-center outside the central region, commonly called “swim”.
An example of distortion can be present in progressive addition lenses (PAL) that possess both far and near correction (add zone for reading) regions and a progressive power change between the two regions. Due to the progressive power change, which is mostly due to change in front or back radius of curvature, there can be distortion around the near region of lens (swim). The progressive power change can create a channel of varying optical power and two swim regions adjacent to this channel. The power change in the channel can possess smooth transition and, in most instances, may not have any distortion. The swim regions can possess distortion due to off-axis astigmatism and other aberrations. The progressive design can be generated on front side, which is typically cast molded, generated front side, back side, or on both sides. Progressive addition lenses can be used by presbyopic patients to provide focusing at distance and near objects without the abrupt change in power.
There is a need for a method for determining a wavefront for a patient's spectacle based on the patient's measured wavefront, in such a way to reduce distortion when the patient looks off-center, outside a central region of the spectacle. There is also a need for developing progressive addition surface (contour map) based on wavefront optimization and weighting functions that are independent of the lens blank base curves for creating both distance region, near region, transition between distance and near regions, and controlling off-axis astigmatism. In other words, what is needed is a method for combining a patient specific low order prescription surface determined from wavefront refraction (sphere, cylinder and axis as derived from low and high order aberration) measured on wavefront aberrometer with wavefront optimized progressive addition surface to create a PAL customized for a particular individual.